Optical Illusions

When we create a drawing (or painting), we must remember that every area of the drawing has an effect on its surrounding areas, so that sometimes a change in one area can create the illusion of a change in another area.

For example, if you draw a square in pencil on white paper, and shade it in with a medium value, you might judge that the square is quite dark.

But if you then surround the square with a very dark background, suddenly it seems much lighter.

Which grey square is darker? In fact, they are exactly the same value

This is because the relationship of values has changed. The original square seemed dark against the white paper, but now it seems quite light relative to the darker background we added.

We need to understand this kind of optical illusion in order to know where our drawing needs to be adjusted.

Sometimes you may be working on a shadow area which seems quite dark, so you try to darken the value in your drawing accordingly, when in fact what you may need to do is to lighten the surrounding values, which makes the shadow appear darker in relation to the lighter values.

Everything is Relative

It’s not just values that have relationships. We need to consider the relationships between edges and spaces too.

For example, when drawing a face: ‘what is the angle of the mouth, relative to the jawline?’

‘How big is the ear shape compared to the length of the nose, what angle does it make, and how far away is it?’

Consider the relationships between edges and shapes as well as values

It’s only by seeing, and accurately describing these relationships, that we will get a convincing drawing.

How Do You Measure Up?

A common way to accurately judge these relationships is by measuring.

There are many different ways of measuring in drawing, but one of the most common is the concept of the basic unit.

The basic unit is a measurement decided upon by the artist, which is used as a reference against which everything else is measured.

If you’ve ever watched artists drawing, you may have been puzzled to see them, from time to time, holding their pencil in an outstretched hand, while closing one eye. What they are doing here is measuring with a basic unit.

Taking the nose length as a basic unit

For example, when drawing a portrait, the width of an eye is often used as a basic unit. Here’s how it’s done:

Hold your pencil out in your fully outstretched arm (to make sure the distance is the same every time), and position the tip of your pencil in line with one corner of the eye on your model (or reference photo).

Now slide your thumb along the pencil, away from the tip, until it is in line with the other corner of the eye.

You now have your basic unit, which is one eye width. Keeping your thumb in position, you can now move around the face, comparing the length of other features to your basic unit.

So you might discover that both eyes are the same width, and there is exactly one eye width between the eyes too. Perhaps the length of the nose is one and a half times the width of the eye, and the ear is the same length as the nose.

Using the eye as a basic unit to measure the other features

Once you have these measurements, you can transfer them to your drawing. Make marks for the corners of the first eye, and then, keeping the measurements you made earlier in mind, make marks for the other eye, the bottom of the nose, the ear etc.

You can then measure the eye you have drawn, again using your pencil and thumb, and then use that measurement to check that your other marks are in the right place, adjusting them accordingly.

Always make your marks first, and then measure and correct if necessary. This will help to train your eye to judge relationships more accurately. If you measure before you make each mark, you will come to rely on the measurements and you won’t learn to judge relationships without measuring.

This is a pretty simplified account of how to use a basic unit for measuring relationships, and there are plenty more techniques you can use, but this covers the basic concept, and should help you to draw more accurately.

As always, let me know in the comments below if you have any questions about relationships in drawing, and I will do my best to help.

When drawing from life or photos the tones on the subject are flat or mixed up, because there are usually to many light sources. It would be better to mentally create one light source and then mentally create the values that will occur on the subject from the imagined single light source. Just my thoughts.